The invention relates to the field of catalysts, in particular zeolites Y of faujasite structure.
The various zeolites are distinguished by different structures and properties. A few structures commonly used in the field of catalysis are described below.
Zeolite Y (FAU) is a large-pore three-dimensional zeolite whose structure has large cavities interconnected by channels formed from 12-membered rings (there are 12 cations (Si4+ and Al3+) and 12 anions O2− present in the ring).
Beta zeolite (BEA) is a large-pore three-dimensional zeolite comprising pores formed from 12-membered rings in all directions.
Zeolite ZMS-5 (MFI) is a medium-pore virtually three-dimensional zeolite, comprising pores formed by 10-membered rings in one direction that are interconnected by zig-zag channels formed by 10-membered rings (it is for this reason that this structure is considered as being virtually three-dimensional).
Mordenite (MOR) is a large-pore zeolite formed by 12-membered rings, with channels extending in only one direction and which has, between these channels, small pockets formed by 8-membered rings.
Ferrierite (FER) is a medium-pore dimensional zeolite comprising main channels formed by 10-membered rings, which are interconnected via side channels formed by 8-membered rings.
Zeolites are important catalytic materials that are widely used in acidic catalytic reactions such as cracking, especially hydrocracking, FCC and olefin cracking, isomerization reactions, especially of paraffins and olefins, and also in methanol conversion technologies, for example MTO, MTP and MTG.
In these reactions, zeolites, and in particular their microporous structures, are often a determining factor for obtaining good catalytic activity, good stability and/or good selectivity.
However, microporous structures may also have drawbacks, for example the poor access of molecules in zeolite crystals, undesired adsorption of reagents and/or of products during catalysis. These steric constraints reduce the accessibility of the microporous volume of the zeolite during the reaction, which may lead in certain cases to sparingly efficient or inefficient use.
Thus, one of the important factors is to obtain zeolites that offer sufficient accessibility to reagents and/or products, in order to improve the efficacy of the catalyst.
Among the envisioned solutions, mention may be made of the reduction of the size of zeolite crystals. However, this solution is not always industrially applicable.
Another strategy consists in creating a system of secondary pores, consisting of mesopores (2-50 nm), in the microporous crystals of zeolites. Traditionally, mesopores are introduced into zeolite or quasi-zeolite crystals by dealumination, for example using a hydrothermal treatment, acidic leaching techniques, or chemical treatments based on EDTA or (NH4)2SiF6.
In recent years, various alternative techniques have been proposed:                recrystallization of the walls of a mesoporous zeolite material,        preparing at the mesoscopic scale a cationic polymer matrix,        constructing a mesoporous material by means of zeolite precursors of organosilicon type, and        direct assembly of zeolite seeds using a matrix to form the mesopores.        
Some of these approaches have led to improved catalysts. However, these techniques are very complex and involve the use of very expensive organic matrices. Thus, the industrial use of these materials is still very limited, especially on account of their very high price.
Moreover, certain prior art techniques may require very specific conditions and/or the use of hazardous and/or pollutant reagents, expensive reagents and/or may not allow mass production.
Finally, certain techniques do not allow good control of the characteristics of catalysts, for example “random” or unoptimized mesoporosity, or alternatively some of the mesopores are cavities, i.e. they are not accessible or not readily accessible from the exterior.
Moreover, an alternative technique for the formation of intracrystalline mesopores has recently consisted of a desilication treatment in alkaline medium.
For example, the publication J. C. Groen et al., Microporous Mesoporous Mater. 114 (2008) 93 describes the alkaline treatment of beta zeolites, performed on the zeolite as prepared, without prior dealumination treatment. At room temperature, virtually no formation of mesopores is observed. Only a higher temperature, of about 318 K, makes it possible to observe the formation of mesopores.
Other publications concern the alkaline treatment of zeolites ZMS-5 (J. C. Groen et al., JACS 127 (2005) 10792) or MFI, BEA, FER and MOR (J. C. Groen et al., Microporous Mesoporous Mater. 69 (2004) 29).
Thus, the invention is directed toward solving all or some of the problems mentioned above.